Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which the invention pertains.
As the rapid evolution of batteries continues, and in particular as secondary electric batteries such as lithium-ion and lithium metal batteries become more widely accepted for a variety of uses, the need for safe, long lasting (greater than 200 cycles) rechargeable cells becomes increasingly important. U.S. Pat. Nos. 5,460,905; 5,462,566; 5,582,623; and 5,587,253 describe the basic elements and performance requirements of secondary lithium batteries and their components. A key issue in the development of high energy secondary batteries is the choice of the electrolyte element to improve the cycle life and safety of the battery.
One of the many problems encountered in the process of producing electrolyte elements is that there is a difficulty in obtaining good cycling efficiency, cycle life, and safety of the cells due to the reactivity of the electrolyte element with the electrode elements, particularly due to reactions with the anode. This is especially true with anodes comprising lithium, which is highly reactive. Reactions of lithium with the electrolyte are undesirable as they lead to self discharge and early battery failure. The reaction of lithium with organic electrolyte solvents may also result in the formation of a surface film on the anode, which subsequently reduces the efficiency of the anode, and may cause uneven plating that can lead to dendrite formation. These factors limit the number of potential electrolyte solvents that may be used for dissolving appropriate electrolyte salts and other additives to form the electrolyte element.
Desirable electrolyte elements provide high cycling efficiency, good ionic conductivity, and reasonable cost. The number of times a lithium battery can be recharged is dependent on the efficiency of each charge and discharge cycle of the cell and provides a measure of the cycling efficiency. By cycling efficiency is meant the percent of the lithium (or other anode material) which is replated or reduced onto the anode upon full charging compared to the amount of lithium freshly stripped or oxidized from the anode on the previous full discharging of the cell. Any deviation in this percentage from 100 percent represents lithium which has been lost in terms of useful availability for the charge/discharge performance of the cell. Cycling efficiency is primarily a function of the ability of the electrolyte solvent to withstand reduction by lithium, which is a powerful reducing agent.
Safety factors affecting the choice of electrolyte solvents include the safety margin against overcharge of the cell. The overcharge safety margin is determined by the voltage difference between completion of recharge of the electrodes and the decomposition of the electrolyte. For instance, in lithium-ion cells, the difference in potential of the anode and cathode is about 4 V. Tarascon and Guyomard, J. Electrochem. Soc., 1991, 138, 2864-2868, describe the upper voltage range of a potential scan being limited to 4.5 V vs. Li/Li.sup.+ because of breakdown of the electrolyte at higher potentials (4.6 V vs. Li/Li.sup.+) in a 1M LiClO.sub.4 50:50 EC (ethylene carbonate):DME (dimethoxyethane) electrolyte. Also, for example, Ein-Eli et al., J. Electrochem. Soc., 1997, 144, L205-L207, report the onset of electrolyte oxidation at 5.1 V for an electrolyte composition comprising 1.2M LiPF.sub.6 ethylene carbonate:dimethyl carbonate (2:3 by volume). The need for electrolyte compositions which do not decompose at high potentials is emphasized by the recent recommendation of Zhong et al., J. Electrochem. Soc., 1997, 144, 205-213, that certain lithium-ion cathode materials should be charged to above 5 V.
Further factors affecting the choice of electrolyte solvents can be illustrated by reference to cells comprising intercalated carbon electrodes. Ein-Eli et al., J. Electrochem. Soc., 1996, 143, L273-277, recently reported that graphite electrodes, which are usually sensitive to the composition of the electrolyte solution, can be successfully cycled at high reversible capacities in electrolytes comprising ethylmethyl carbonate. These results are interesting because lithium ions cannot intercalate into graphite in diethyl carbonate solutions and cycle poorly in dimethyl carbonate solutions.
A large number of non-aqueous organic solvents have been suggested and investigated as electrolytes in connection with various types of cells containing lithium electrodes. U.S. Pat. Nos. 3,185,590; 3,578,500; 3,778,310; 3,877,983; 4,163,829; 4,118,550; 4,252,876; 4,499,161; 4,740,436; and 5,079,109 describe many possible electrolyte element combinations and electrolyte solvents, such as borates, substituted and unsubstituted ethers, cyclic ethers, polyethers, esters, sulfones, alkylene carbonates, organic sulfites, organic sulfates, organic nitrites and organic nitro compounds.
One class of organic electrolyte solvents that have received attention as a component of electrolyte elements for electrochemical cells and other devices are the sulfones. Sulfones can be divided into two types: the cyclic or aromatic sulfones, commonly referred to as sulfolanes; and the aliphatic sulfones. Sulfones form a potentially attractive group of organic solvents which present a high chemical and thermal stability.
The use of the cyclic sulfones, sulfolane (tetramethylenesulfone) along with its alkyl-substituted derivatives, 3-methylsulfolane and 2,4-dimethysulfolane, as electrolyte solvents has been investigated.
U.S. Pat. No. 3,907,597 to Mellors describes a liquid organic electrolyte consisting essentially of sulfolane or its liquid alkyl-substituted derivatives in combination with a co-solvent, preferably a low viscosity solvent such as 1,3-dioxolane, and an ionizable salt. Sulfolane and its liquid alkyl-substituted derivatives, such as 3-methyl sulfolane, are good non-aqueous solvents but have the disadvantage in that they have a relatively high viscosity. Thus, when metal salts are dissolved in these solvents for the purpose of improving the ionic conductivity of the solvents, the viscosity of the solvent and the salt becomes too high for its efficient use as an electrolyte for non-aqueous cell applications. For example, in the '597 patent, sulfolane is used in combination with a low viscosity co-solvent to overcome the viscosity problem.
Japanese patent publications numbers JP 08-298229, published Nov. 12, 1996 and JP 08-298230, published Nov. 12, 1996, describe electrolytes for electric double layer capacitors which comprise sulfolane as one of the electrolyte components.
U.S. Pat. No. 4,725,927 to Morimoto et al. describes the use of sulfolane and its derivatives, 3-methylsulfolane and 2,4-dimethylsulfolane, for use in electric double layer capacitors. However they note that a sulfolane solvent has a high viscosity and a relatively high solidification temperature. Therefore, when it is used for an electrolyte solution, the ionic conductivity tends to be low.
U.S. Pat. No. 5,079,109 to Takami et al. describes a non-aqueous electrolyte solvent blend that may comprise sulfolane as one of the components for use in rechargeable lithium secondary batteries. U.S. Pat. No. 5,219,684 to Wilkinson et al. describes an electrolyte consisting essentially of sulfolane and a glyme for an electrochemical cell comprising a lithium containing anode and a cathode, including Li.sub.x MnO.sub.2 cathode active material.
U.S. Pat. No. 4,550,064 to Yen et al. describes electrolytes with sulfolane type solvents which have relatively high dielectric constants and low vapor pressure. Electrolytes containing sulfolane also exhibit improved stripping/plating cycling efficiency because of the excellent reduction stability. However, the use of sulfolane solvents is inhibited by incompatibility of the polar sulfolane liquid with the hydrophobic separator and with the non-polar binder of the cathode. Methods to improve the wettability of the separator and the cathode electrode are described.
The use of the aliphatic sulfones, dimethylsulfone and dipropylsulfone, has been investigated as electrolyte solvents. U.S. Pat. No. 4,690,877 to Gabano et al. reports electrolyte compositions containing at least one aromatic or aliphatic linear sulfone for use in cells operable at temperatures between 100.degree. C. and 200.degree. C. Particularly preferred was dimethylsulfone.
Sulfone-based electrolytes comprising dimethylsulfone, dipropylsulfone, and sulfolane have been described by J. Pereira-Ramos et al., J. Power Sources, 1985, 16, 193-204 for use in lithium intercalation batteries. Molten dimethylsulfone at 150.degree. C. as an electrolyte for a rechargeable .gamma.-MnO.sub.2 lithium battery is described by Bach et al., J. Power Sources, 1993, 43-44, 569-575.
U.S. Pat. Nos. 4,060,674 and 4,104,451 to Klemann and Newman describe electrolyte compositions for reversible alkali metal cells which consist essentially of a solvent and an electronically active alkali metal salt. Organic electrolyte solvents employed are generally ones selected from the group consisting of inertly substituted and unsubstituted ethers, esters, sulfones, organic sulfites, organic sulfates, organic nitrites or organic nitro compounds. Examples of organic solvents include propylene carbonate, tetrahydrofuran, dioxolane, furan, sulfolane, dimethyl sulfite, nitrobenzene, nitromethane and the like. The preferred solvents are ethers, and preferred is an electrolyte solvent containing dioxolane.
JP patent publication number JP 09-147913, published Jun. 6, 1997, describes electrolyte solvents containing sulfones of the formula R.sup.1 --SO.sub.2 --R.sup.2, where R.sup.1 and R.sup.2 are C.sub.1-4 alkyl groups, and R.sup.1 and R.sup.2 are different. Preferably the anodes are Li interaction carbonaceous anodes.
Despite the numerous electrolyte solvents proposed for use in rechargeable cells, there remains a need for improved non-aqueous electrolyte solvents that provide beneficial effects during the useful life of the cell, and which can be incorporated easily and reliably into the cell without significant extra cost.
It is therefore an object of the present invention to provide an improved non-aqueous electrolyte solvent which is suitable for use in rechargeable cells.
It is yet a further object of the present invention to provide a non-aqueous electrolyte solvent which has greater overcharge safety margins.
It is yet a further object of the present invention to provide a non-aqueous electrolyte solvent with high ionic conductivity and low solvent volatility.
It is a further object of this invention to provide an improved non-aqueous electrolyte solvent for electrochemical cells which comprise alkali metal negative electrodes.
Yet another object of the present invention is to provide a non-aqueous electrolyte solvent that is useful with both lithium metal and lithium-ion anodes for secondary battery cells.
It is a further object of the present invention to provide a non-aqueous electrolyte solvent that provides for reversible intercalation of lithium into graphite.
It is a further object of the present invention to provide a non-aqueous electrolyte solvent that increases the cycle life and safety of secondary cells.
It is yet a further object of the present invention to provide secondary lithium cells employing the electrolytes of the present invention and methods of making such cells.